A barren Class D planetoid in the Mutara sector, orbited by the laboratory Regula I and hollowed out for the second stage of the Genesis experiment.
Regula is the planetoid seen in Star Trek II: The Wrath of Khan. It is a dead lump of rock with no atmosphere worth the name, chosen by Dr Carol Marcus and her team because nothing lives there. The space laboratory Regula I orbits it.
For stage two of Project Genesis the scientists spent months tunnelling into the planetoid and then used the Genesis effect in a cavern underground. The result is a huge cave full of plants, water and light.
Carol and David Marcus hide there with the device when Khan attacks the station. Kirk's party follows them down, Khan beams the device away and leaves Kirk apparently buried alive, prompting his roar of rage. Kirk has already arranged in code for the Enterprise to return and collect them.